Device for performing an assay, a method for manufacturing said device, and use of a membrane in the manufacture of said device

ABSTRACT

A device for performing an assay comprising a substrate having through-going channels that open out onto a surface for sample application, the channels being provided in at least one cross-sectional area with a first binding substance capable of binding a particular analyte, the substrate being an electrochemically manufactured metal oxide membrane and containing the first binding substance within the through-going channels.

The present invention relates to a device for performing an assay, whichdevice comprises a substrate having oriented through-going channels,said channels opening out on a surface for sample application, thechannels in at least one area of the surface for sample applicationbeing provided with a first binding substance capable of binding to ananalyte.

BACKGROUND OF THE INVENTION

Such a device is disclosed in WO95/11755 for “sequencing byhybridisation” applications. The device comprises a substrate providedwith channels, the channels being oriented substantially perpendicularto the surface of the substrate. Three types of substrate are disclosed.The first type is comprised of a multitude of hollow glass fibres. It ismanufactured by stacking glass fibres having an etchable core, providingthe stack with flat ends, polishing those ends, and etching the cores,usually with acid. The second type of substrate is produced byelectrochemical etching of a crystalline silicon wafer. First, theposition of the channels as well as their size are defined usingstandard photolithographic methods. Subsequently the oriented channelsare formed electrochemically. The third type of substrate is produced bynuclear track etching of an inorganic substrate. This method, comprisingthe steps of exposing the substrate to heavy, energetic chargedparticles and wet-etching, results in a substrate with channelsscattered randomly over the surface of the substrate. With higher poredensities and porosity there is more chance of fusion of channels, whichshow reduced flow resistance with respect to other, non-fused channels.

All three types of substrates are quite expensive because of thelabour-intensive manufacturing processes and/or expensive startingmaterials and wasteful operations, such as sawing and polishing, and/orexpensive equipment. In addition, the substrates are characterised by arelatively low porosity of 30% and more. More advantageous, higherporosities of up to 80% are said to be achievable, but only atrelatively low channel densities, with the disadvantage that theeffective surface area of the channels of a particular area of thesubstrate is lower in comparison with a substrate having a comparableporosity but with higher channel densities (and consequently narrowerchannels). A further disadvantage of the silicon-based substrates asdisclosed in WO 95/11755 is that they are not transparent for light.These substrates therefore prohibit the advantageous use of opticalmarker systems for the detection of analyte bound in the substrate.Popular optical marker systems are for instance based on enzymaticallyinduced colour reactions, on bio- or chemi-luminescence, or onphotoluminescence. In the latter case both the excitation light andemitted luminescent light have to pass through the substrate material.

The object of the present invention is to overcome the abovedisadvantages and provide a substrate having both a high channel densityand a high porosity, allowing even higher density arrays comprisingdifferent first binding substances per unit of the surface for sampleapplication. In addition, the substrate is highly transparent forvisible light. More in particular, the object of the present inventionis to provide a device comprising a relatively cheap substrate that doesnot require the use of any typical microfabrication technology and, thatoffers an improved control over the liquid distribution over the surfaceof the substrate.

The above objects are achieved with a device wherein the poroussubstrate is an electrochemically manufactured metal oxide membrane.

Metal oxide membranes having through-going, oriented channels can bemanufactured cheaply through electrochemical etching of a metal sheet.Metals considered are, among others, tantalum, titanium, and aluminium,as well as alloys of two or more metals and doped metals and alloys. Themetal oxide membranes are transparent, especially if wet, which allowsfor assays using various optical techniques. Such membranes haveoriented channels with well controlled diameter and advantageouschemical surface properties.

SUMMARY OF THE INVENTION

The invention thus provides a device for performing an assay, whichdevice comprises a substrate having oriented through-going channels,said channels opening out on a surface for sample application, thechannels in at least one area of the surface for sample applicationbeing provided with a first binding substance capable of binding to ananalyte, wherein the substrate is an electrochemically manufacturedmetal oxide membrane.

DETAILED DESCRIPTION OF THE INVENTION

According to a preferred embodiment, the first binding substance ischosen from the group consisting of a nucleic acid probe, an antibody,an antigen, a receptor, a hapten, and a ligand for a receptor.

Assays in which the device according to the present invention can beused may include sequencing by hybridisation, immunoassays,receptor/ligand assays and the like.

When the device is used as a tool to obtain DNA sequence information, alarge array of areas is provided, each area comprising as a firstbinding substance an oligonucleotide probe of a different base-pairsequence. If a sample containing DNA or RNA fragments with a (partly)unknown sequence is brought into contact with the substrate a specifichybridisation pattern may occur, from which pattern the sequenceinformation of the DNA/RNA can be derived. Such “sequencing byhybridisation” methods are well known in the art (see e.g. Fodor, S.P.A.et al. (1992), Science 251, 767-773 and Southern, E.M.et al. (1994)Nucleic Acids Res. 22, 1368-1373).

The device according to the present invention may also be used to screena biological specimen, such as blood, for a large number of analytes.The array may consist of areas comprising oligonucleotide probesspecific for, for example, E. coli, S. aureus, S. pneumoniae etc. Abiological sample can be prepared as described in EP 0.389.063. If thissample is brought into contact with the substrate, the resultinghybridisation pattern can be read e.g. using a CCD camera in combinationwith an appropriate optical marker. Apart from screening for bacteria,the device is suitable for the detection of viruses, as well as theclassification of different subtypes of, for example, HIV- and HCVviruses, etc. Virus classification may be essential to determinepotential drug resistance. In general it requires the ability to detectsingle point mutations in the virus RNA.

The device is also suited for performing sandwich immunoassays. In thatcase, it is preferred that a second antibody is used for binding tobound analyte, said second antibody for each of the analyte beingrecognised by a third labelled antibody. This may be achieved if thesecond and third antibodies are derived from different species and thethird antibody is raised against antibodies of the other species. Thusit is avoided to label the second antibody for each particular analyte.

The device is also suited for performing “pepscans” as disclosed inGeysen et al., Proc. Natl. Acad. Sci. USA 81:3998-4002 (1984). In thatcase the first binding substances that are attached to the differentareas of the substrate constitute different sequences of aminoacids. Ifthe substrate is brought into contact with a liquid that contains aparticular analyte, a reaction pattern may occur representing thespecific affinity of the analyte for the different aminoacid sequences.

It is preferred that the first binding substance is covalently bound tothe substrate. This minimises loss of the first binding substance fromthe substrate. Covalent binding of an organic compound to a metal oxideis well known in the art, for example using the method described by Chu.C. W., et al. (J. Adhesion Sci. Technol., 7, pp.417-433, 1993) andFadda, M. B. et al. (Biotechnology and Applied Biochemistry, 16, pp.221-227, 1992).

According to a preferred embodiment the metal oxide membrane iscomprised of aluminium oxide.

Such a membrane of aluminium oxide appears to have through-goingchannels that are hydrophilic in comparison to the surface of themembrane. Thus, advantageously, a hydrophilic liquid preferably entersthe channels instead of spreading over the surface of the membrane.Therefore aluminium oxide membranes may accommodate for high densitiesof areas comprising different first binding substances. Aluminium oxidemembranes having oriented through-going channels are disclosed by Rigby,W. R. et al. (Trans. Inst. Metal Finish., 68(3), p. 95, 1990) and aremarketed by Anotec Separations Ltd., Oxon, UK. These membranes have beenused to purify viruses, and to store enzymes for sensor purposes, butthere is no suggestion with respect to their suitability as substratesfor performing probe-based assays.

The present invention also relates to a method of manufacturing a devicecomprising membranes having oriented through-going channels according tothe invention, wherein the first binding substance is synthesised insitu. For example, using only a limited number of reagents, for a devicecomprising an oligonucleotide as the first binding substance usuallyfour nucleotide compounds (dA, dT, dC, and dG for DNA, A, U, C, and Gfor RNA) and additional reagents such as blocking reagents, andprotecting reagents, classical solid phase synthesis techniques can beused to provide a substrate with one or an array of a plurality of areaswith oligonucleotide probes. Reagents can conveniently be applied to thethrough-going channels of a particular area using ink-jet technology.Ink-jet technology allows for the accurate deposition of defined volumesof liquid. In situ synthesis of oligonucleotide probes on a flat,non-porous substrate is well known in the art (see eg. T. P.Theriault:DNA diagnostic systems based on novel Chem-Jet technologies, IBCConference on Biochip Array Technologies, Washington D.C., May 10 1995).

According to a preferred embodiment, the nucleotide compounds areapplied using electrostatic attraction. Electrostatic attractiondiminishes the risk of splattering.

According to an alternative method of manufacturing a device comprisingthrough-going channels according to the invention, the first bindingsubstance is applied to the through-going channels of a particular areausing ink-jet technology. This allows for purification of the firstbinding substance, and for example in case of an oligonucleotide probefor verification of its sequence, before application to the substrate.

For the reasons mentioned earlier, it is again preferred if the firstbinding substance is applied using electrostatic attraction.

The present invention also relates to the use of an electrochemicallymanufactured metal oxide membrane, preferably an aluminium oxidemembrane, in the manufacture of any of the above described devices.

According to a preferred embodiment, a temperature difference isadjusted between different locations on the membrane during performanceof the assay to create different hybridisation conditions at differentmembrane locations.

The use advantageously comprises a nucleic acid hybridisation assay oran immunological assay. In such an assay, a sample which comprises ananalyte is brought into contact with a device according to theinvention. The analyte is subsequently allowed to bind to the firstbinding substance which is attached to the substrate. Such binding isgreatly facilitated by allowing the analyte to migrate through theporous substrate. Detection of binding can be performed by adding asecond binding substance attached to a label, allowing said secondbinding substance to bind to the complex of first binding substance andanalyte and determining whether the label is present at the positionwhere the first binding substance was immobilised. Alternatively, theanalyte may already have been provided with a label, in which casebinding to the first binding substance can be detected directly, withoutthe addition of a second binding substance.

The present invention also relates to a kit comprising any of the abovementioned devices which kit additionally comprises a detection means fordetermining whether binding has occurred between the first bindingsubstance and the analyte. Preferably, such detection means may be asecond binding substance provided with a label. Preferably, the label iscapable of inducing a colour reaction and or capable of bio- or chemo-or photoluminescence.

The present invention also relates to a method for the detection of ananalyte in a sample comprising the steps of

a) contacting the sample with any of the above described devices,

b) allowing binding to take place between the first binding substanceand the analyte,

c) detecting whether binding has occurred between first bindingsubstance and analyte

In this method the analyte may be a nucleic acid probe, an antibody, anantigen, a receptor, a hapten, and a ligand for a receptor.

The present invention will now be illustrated by the following examples.

EXAMPLE 1

Simultaneous detection of two different types of HIV-1 amplificate, aWild Type RNA (WT) and a Calibrator RNA (Qa) using an aluminium oxidemembrane in a flow through cell.

Analytes:

The WT-RNA and the Qa-RNA fragments represent a part from the GAG regionof the HIV-1 genome. These fragments have equal lengths (145nt) andidentical sequences, apart from a 21nt long region in the central partof the fragment.

The sequences of the fragments are:

WT-RNA: 5′cccugcuaugucacuuccccuugguucucucaucuggccuggugcaauaggcccugcaugcacuggaugcacucuaucccauucugcagcuuccucauugauggucucuuuuaacauuugcauggcugcuugau guccccccacu3′ (SEQID.NO.1)

Qa-RNA: 5′cccugcuaugucacuuccccuugguucucucaucuggccuggugcaauaggcccugcaugcgacugucaucuaucuacacugucugcagcuuccucauugauggucucuuuuaacauuugcauggcugcuuga uguccccccacu3′ (SEQID.NO.2)

The sequence of the WT and Qa specific parts are underlined.

In this example two buffered solutions were used:

A phosphate buffer at pH 7.4 containing 8 g/l NaCl, (“incubationbuffer”).

A phosphate buffer at pH 7.4 containing 8 g/l NaCl and 0.05% Polysorbate(Tween 20), hereinafter denoted “wash buffer”.

Substrate:

Aluminium oxide membrane, thick 60 μm, diameter 24 mm. Channels are 0.2μm diameter, density is about 18 channels/μ² (“Anodisc 25”, Whatman).

The membrane surface is coated with streptavidin by immersing themembrane in the incubation buffer contained 2 g/l streptavidin for 60min. Subsequently, the membranes are washed using the wash buffer andair dried at room temperature.

Immobilisation of First Binding Substance

Two oligonucleotide probes, partially complementary to the WT- and QAfragments are applied:

WT-probe: 5′GAATGGGATAGAGTGCATCCAGTG3′ (SEQID. NO. 3)

Qa-probe: 5′GACAGTGTAGATAGATGACAGTCG3′ (SEQID. NO. 4)

both with a biotin molecule coupled to the 5′ end.

Spots with a specific diameter are applied using a porous tip (nylonfeeder) as found in the common “fineliner” writing pen (Hauserschreibtechnik GmbH,. Gosheim Germany). Whereas the feeder tip spots themembrane, its other end is in fluid contact with a reservoir containingthe probe solution (incubation buffer, probe concentration 25 μmol/L).Transfer of probe solution into the membrane is well controlled by thecapillary interaction of membrane and feeder: the probe solutionautonomously fills up those channels that are in physical contact withthe feeder tip. In this example 2 lines with 3 spots of 0.5 mm diameterhave been used (3 spots for each probe type). The distance betweenindividual spots was 1 mm. After spotting and an incubation phase of 10min. at room temperature, unbound probe material is washed away usingthe wash buffer.

In this example, 4 identical substrates were produced in this way.

Hybridisation

Next, the membranes are introduced in a flow through cell and broughtinto contact with the incubation buffer containing the HIV RNAfragments.

Four sets of hybridisation conditions have been applied in 4 differentexperiments:

1 volume 25 μl containing 1.5*10¹² molecules of QA RNA, no flow

2 volume 25 μl containing 1.5*10¹² molecules of WT RNA, no flow

3 volume 25 μl containing 1.5*10¹² molecules of QA RNA, continuous flow

4 volume 25 μl containing 1.5*10¹² molecules of WT RNA, continuous flow

With experiment 1 and 2 there is no transport of the buffer through themembrane. With experiment 3 and 4, the 25 μl RNA solution continuouslyflows through the membrane in two directions (back and forth) with avelocity of about 25 μl/min. To control this flow, an automated Hamiltondispenser was used.

With all experiments hybridisation was at room temperature during 10min.

Washing

After hybridisation the membranes are washed using 5ml of the washbuffer.

Labelling and Detection

For detection, a probe that is generic for HIV RNA (SEQID #5) is allowedto interact with the membranes. This probe is contained in theincubation buffer (40 nmol/L). In each experiment a volume of 75 μl isused, without flow. The probes are labelled with the horseradishperoxidase (HRP) enzyme in a 1:1 ratio, using maleimide containingheterobifunctional cross-linkers (Hashida,S., et al.(1984) J. AppliedBiochem.56, 56-63). Prior to the HRP coupling the probes were thiolated(Carlsson, J., et al. (1978) Biochem. J. 173, 723-737).

After washing with 10 ml wash buffer, a solution containing3,3′,5,5′-tetramethylbenzidine hydrogenperoxide, TMB (Organon Teknika,art: 78510), is brought into contact with the membranes (no flow).

Result:

Interpretation of the results was with the unaided eye. In experiment 3and 4, blue spots appear almost immediately at a location where aspecific reaction is expected (spots containing WT probes turn blueusing WT-RNA and spots containing Qa probes turn blue using Qa-RNA).With the spots containing probes that are not complementary to the RNAin the buffer no colouring was observed, although the area on themembrane in between the spots shows a slight bluish colour after severalminutes, probably due to insufficient washing or some non specificbinding. In experiment 1 and 2 a similar result is obtained, however, inthese cases it takes about a minute before blue spots become visible.

In addition to the visual evaluation of the spots during the TMBreaction, the spots on the membranes in experiments 3 and 4 wereevaluated using an imaging densitometer (Biorad GS700). To this end themembranes were removed from the flow-through cells (Table 1)

TABLE 1 Density of spots measured with densitometer spot with WT- spotwith Qa- background area RNA analyte probes [OD units] probes [OD units][OD units] WT-RNA 38 20 20 Qa-RNA 25 35 25

EXAMPLE 2

Oligonucleotide probes were covalently coupled to the Anopore membranesusing 3-aminopropyl triethoxysilane (APS) as a linker between thealumina and the oligo. For the experiments Anodics 25 membranes with adiameter of 25 mm and a total surface area of 0.3 m² were used.

The membranes were activated by immersion in a nitric acid solution (0.4mol/l) during 1 hour. After rinsing with water the membranes were driedand immersed in a 0.25% (v/v) solution of APS in water for 2 hours.Excess APS was removed by rinsing with water. After drying at 120 μ° C.at reduced pressure the membranes were stored. Amino group concentrationdue to the coupling of the APS molecules was typical 2-3 μmol/m².

Before coupling, the amino group terminated oligo nucleotides wereactivated by reaction with disuccinimidyl suberate (DSS, see e.g. PIERCEBV, Immunotechnology Catalog & Handbook, 1990). The resultingsuccinimidyl group at the end of the oligo was used for coupling to theAPS activated membrane. Labelling with ³²p was used for quantificationof the results. Coupling with 500 ul oligo solution on an Anodiscmembrane during 60 minutes resulted in a coupling yield of 110⁻¹⁰ mol/m²oligo nucleotide.

EXAMPLE 3

Definition of an array pattern on an Al₂O₃ membrane using an ink-jetdevice. Using standard ink-jet technology small droplets having adiameter of 20-80 μm can be generated and positioned on a substrate athigh throughput rates at μm resolution. Using a commercially availabledesk-jet (HP 660C) in combination with the Al₂O₃ membranes arrays of avery high resolution have been obtained. Visual inspection with amicroscope (magnification: 400×) shows perfectly round spots of aprox.60 μm diameter having very sharp margins. No signs of splattering, as iscommonly observed when using non-porous surfaces was observed. Weattribute the high array resolution to the high porosity of the materialin combination with the hydrophilic character of the through-goingchannels.

EXAMPLE 4

Performing a Sandwich Immuno Assay.

Detection of human chorionic gonadotrophin (hCG) with an enzyme immunoassay using an aluminium oxide membrane as solid phase.

Coating of the Membrane

Small areas of aluminium oxide membranes (round with a diameter of 20mm) were coated with a buffered solution (0.0127 mol/l phosphate and0.140 mol/l NaCl at pH 7.4) containing 1 μg/ml monoclonal mouse antibody(OT-hCG-4B) directed against hCG. The solution was applied by pipetting10 ul droplets onto the membrane or by contact spotting using apolyester feeder (Hauser). After incubation at 37° C. for 30 minutes themembranes are ready for use.

Incubation

The positive samples were a mixture of 50 ul hCG with a concentration of2000 IU/I and 50 ul mouse anti-hCG (OT-hCG-3A) conjugated with horsradish peroxidase (HRP) (1 μg/ml). This mixture was pre-incubated for 15minutes. In the case of the negative control 50 μl buffer was mixed with50 μl conjugate solution.

Next the mixture (100 ul) was pipetted onto the membranes and incubatedfor 15 minutes at room temperature.

Washing and Detection

The membranes were extensively rinsed with a washing buffer (0.131 mol/lNaCl, 0.0127 mol/l phosphate and 0.5 ml/I Polysorbate 20) on a funnel.Finally the membranes were placed in a beaker containing a substrate forHRP based on 3,3′,5,5′- tetramethylbenzydine and hydrogen peroxide(Organon Teknika). During 30 minutes incubation the results wereobserved visually and with a camera.

Results

Clear blue spots became visible within a few minutes where the membraneswere coated with OT-hCG-4B in the case of the positive samples. On theother parts of the membrane and with the negative control only a faintblue background colour could be observed after relative long incubation.

5 1 145 RNA Human immunodeficiency virus type 1 1 cccugcuaug ucacuuccccuugguucucu caucuggccu ggugcaauag gcccugcaug 60 cacuggaugc acucuaucccauucugcagc uuccucauug auggucucuu uuaacauuug 120 cauggcugcu ugauguccccccacu 145 2 145 RNA Human immunodeficiency virus type 1 2 cccugcuaugucacuucccc uugguucucu caucuggccu ggugcaauag gcccugcaug 60 cgacugucaucuaucuacac ugucugcagc uuccucauug auggucucuu uuaacauuug 120 cauggcugcuugaugucccc ccacu 145 3 24 DNA Human immunodeficiency virus type 1 3gaatgggata gagtgcatcc agtg 24 4 24 DNA Human immunodeficiency virus type1 4 gacagtgtag atagatgaca gtcg 24 5 25 DNA Human immunodeficiency virustype 1 5 tgttaaaaga gaccatcaat gagga 25

What is claimed is:
 1. A device for performing an assay, which devicecomprises a substrate having through-going channels, said channelsopening out on a surface and being oriented essentially perpendicular tothe surface for sample application, the channels in at least one area ofthe surface for sample application being provided with a first bindingsubstance capable of binding to an analyte, wherein the substrate is anelectrochemically manufactured metal oxide membrane and the firstbinding substance is within the through-going channels in the substrate.2. The device according to claim 1, wherein the first binding substanceis chosen from the group consisting of a nucleic acid probe, anantibody, an antigen, a receptor, a hapten and a ligand for a receptor.3. The device according to claim 1, wherein the first binding substanceis covalently bound to the substrate.
 4. The device according to claim1, wherein the metal oxide membrane is comprised of aluminium oxide. 5.A method of manufacturing a device according to claim 1, wherein thefirst binding substance is synthesised in situ.
 6. The method accordingto claim 5, wherein a compound for synthesising the first bindingsubstance is applied to a particular area using ink-jet technology. 7.The method according to claim 6, wherein the compound is applied usingelectrostatic attraction.
 8. A method of manufacturing a deviceaccording to claim 1, wherein the first binding substance is applied toa particular area using ink-jet technology.
 9. The method according toclaim 8, wherein the first binding substance is applied usingelectrostatic attraction.
 10. A kit comprising a device according toclaim 1, and a detection means for determining whether binding hasoccurred between the first binding substance and the analyte.
 11. Thekit according to claim 10 wherein the detection means comprises a secondbinding substance provided with a label.
 12. The kit according to claim11, wherein the label is capable of inducing a color reaction or capableof bio- or chemo- or photoluminescence.
 13. A method for the detectionof an analyte in a sample, comprising the steps of a) contacting thesample with a device according to claim 1, b) allowing binding to takeplace between the first binding substance and the analyte to bedetected, and c) detecting whether binding has occurred between firstbinding substance and analyte.
 14. The method of claim 13, wherein theanalyte comprises nucleic acid.
 15. The method of claim 14, wherein thenucleic acid is derivable from human immunodeficiency virus.